For the polymerisation of butadiene, it is known from US patent specification U.S. Pat. No. 3,794,604 (see examples of embodiment) to use a catalytic system of “preformed” type in the presence of a conjugated diene monomer, comprising:
butadiene or isoprene as the conjugated diene monomer,
cerium octanoate as the rare earth salt in solution in benzene,
diisobutylaluminium hydride as the alkylating agent, in an “alkylating agent:rare earth salt” molar ratio substantially equal to 20, and
ethylaluminium dichloride as the alkylaluminium halide.
Japanese patent specification JP-A-60/23406 also describes a catalytic system of the type which has been “preformed” in the presence of butadiene, the system specifically being intended for the polymerisation of butadiene. The catalytic systems of this document comprise:
butadiene as the conjugated diene monomer,
a rare earth salt of a trivalent or pentavalent organic phosphoric acid in solution in n-hexane or cyclohexane,
an alkylating agent consisting of an alkylaluminium of formula Al(R5)3-nHn, where n has the value 0, 1 or 2 and R5 is a hydrocarbon comprising 1 to 8 carbon atoms, and
a halogenated Lewis acid belonging equally well to the family of monohalides, sesquihalides and oligohalides [of] elements of groups IIIa, Iva or Va of Mendeleev's periodic table or alternatively organometallic alkylaluminium halides.
In virtually all the examples of embodiment of this document, the halogenated Lewis acid which is effectively used is an alkylaluminium sesquichloride and, in these examples, the corresponding catalytic systems comprise said rare earth in a concentration varying substantially from 0.0002 mol/l to 0.016 mol/l (see Example 23, in which the estimated concentration of rare earth is between 0.015 and 0.016 mol/l).
In other examples of embodiment (Examples 10 and 19), the halogenated Lewis acid is diethylaluminium chloride or ethylaluminium bromide and the corresponding catalytic systems then comprise said rare earth in the extremely reduced concentration of 0.0002 mol/l.
As for the “alkylating agent:rare earth salt” molar ratios which are effectively used in these examples of embodiment, they vary from 10 to 30.
One major disadvantage of all the catalytic systems tested in this patent specification JP-A-60/23406 lies in the fact that none of the polybutadienes obtained by means of these catalytic systems has both a polydispersity index of less than 2.1 and a Mooney viscosity ML(1+4) at 100° C. at least equal to 40. As a result, these polybutadienes are not suitable for use in a tyre tread.
Another drawback of the catalytic systems of this document JP-A-60/23406 lies in the non-reproducibility of the microstructure characteristics of the polybutadienes obtained, in particular for the cis-1,4 linkage contents, which may vary significantly from 89% to 99% in the examples of embodiment.
The applicant has unexpectedly discovered that a catalytic system of the “preformed” type based on at least:
a conjugated diene monomer,
an organic phosphoric acid salt of one or more rare earth metals (metals with an atomic number between 57 and 71 in Mendeleev's periodic table), said salt being in suspension in at least one inert, saturated and aliphatic or alicyclic hydrocarbon solvent, and this catalytic system comprising the rare earth metal or metals in a concentration equal to or greater than 0.005 mol/l,
an alkylating agent consisting of an alkylaluminium of formula AlR3 or HAlR2, the “alkylating agent:rare earth salt” molar ratio being greater than 5, and
a halogen donor which belongs to the family of alkylaluminium halides with the exception of alkylaluminium sesquihalides,
makes it possible to overcome the aforementioned drawbacks by leading in particular to obtaining polybutadienes having, on one hand, a polydispersity index, measured by the “SEC” technique of size exclusion chromatography (see attached Appendix 2), which is less than 2.1 and, on the other hand, a Mooney viscosity ML(1+4) at 100° C., measured in accordance with Standard ASTM D 1646, which may be equal to or greater than 40. These combined characteristics make the polybutadienes obtained by means of catalytic systems according to the invention particularly well-suited for use in tyre treads.